1. Field of Invention
This invention relates to apparatus for removing substances deposited on rolls in molten metal plating baths. This invention also relates to methods for preventing the occurrence of dents formed on molten metal plated metal strips.
2. Description of Related Art
In a continuous molten zinc plating line for galvanizing a traveling steel strip (also called steel sheet) by continuously dipping the strip in a molten zinc plating bath, molten zinc deposited on surfaces of the steel strip having been pulled up from the molten zinc plating bath is usually wiped by blowing gas (called wiping gas) against the surface of the steel strip through gas blowing nozzles called wiping nozzles in order to adjust the amount of the molten zinc deposited on surfaces of the steel strip. For this purpose, the steel strip is dipped in the molten zinc plating bath (hereinafter, referred to as a plating bath) obliquely downward and is pulled upward from the plating bath by changing the traveling direction of the steel strip to a vertical direction. The traveling direction of the steel strip is ordinarily changed by a rotary body (roll) called a sink roll dipped in the plating bath.
A large amount of foreign substances called dross are ordinarily produced in the plating bath by the precipitation and oxidation of Znxe2x80x94Fe alloy, Znxe2x80x94Al alloy and the like, and deposited on surfaces of the sink roll. Then, when the dross has grown to a certain size, it comes into contact with the steel strip traveling on the sink roll and causes defects of dents on surfaces of the steel strip.
In particular, while alloyed-molten-zinc-plated steel sheet has recently been used as steel sheet for automobiles in a large quantity, automobile manufacturers require more severe quality for steel sheet and even a minute defect on the plated steel sheet is not permitted. Accordingly, the defects of dents caused by the deposition of dross on the surface of the sink roll are a very serious problem to steel sheet suppliers.
Countermeasures for this problem have been studied. For example, a technology for thermal spraying ceramic, on which it is difficult to deposit dross, onto the surface of the sink roll has been proposed. However, the dross deposited on the surface of the sink roll cannot be perfectly prevented by this technology.
Thus, a method for mechanically removing foreign substances on the surface of the sink roll has been developed and is in practical use. Specifically, the method removes foreign substances by scraping them away by pressing a blade, which has a length entirely covering the surface of a sink roll in its axial direction, against the surface of the sink roll.
The blade must be in uniform contact with the entire surface of the sink roll in its axial direction in the conventional technology, which is very difficult to achieve, and the surface of the sink roll has not yet been put in a satisfactory contact state. In particular, the blade has such a defect that it is worn by use, which makes it more difficult for the blade to be in uniform contact with the surface of the sink roll. Further, when the blade is unevenly worn, the overall blade must be replaced to ensure uniform contact of the blade, from which the problem arises that the cost of the blade is increased.
To cope with the above problem, the applicant previously proposed a method for scraping away deposited substances by moving a narrow blade along the axis of a sink roll in Japanese Unexamined Patent Application Publication No. 61-133369. At that time, the applicant also proposed to control the press force of a drive unit (composed of a worm gear, a worm wheel, a screw shaft and the like) for moving the blade forward and backward with respect to the surface of the sink roll, so as to adjust the press force to a proper value by mounting a sensor (torque sensor) for measuring the press force on the blade in order to uniformly press the blade against the sink roll. The reason that the press force is measured is that the sink roll is not generally rotated by a motor and rotated by a steel strip that travels in contact with the sink roll. Thus, an excessively large press force stops the sink roll and operation cannot be smoothly carried out.
However, this technology requires a torque sensor and the blade drive unit, which makes the apparatus more complex and expensive than desired. Further, a problem also arises that a press force controller must be installed just above a molten zinc plating bath. The technology has the disadvantage that the press force controller needs to be exposed to a very bad environment and the adjustment, maintenance and inspection of the press force becomes difficult because the above-mentioned gas wiping nozzle for adjusting the amount of plated molten zinc deposited on a steel strip, and the like are disposed in the vicinity of the press force controller and the molten metal plated on the steel strip is partly scattered by the gas used for the wiping nozzle.
In view of the above-described circumstances, it is an object of this invention to provide an apparatus for removing substances deposited on a roll in a molten metal plating bath. The apparatus is installed in a molten metal plating line. The apparatus can provide a simple structure, is less expensive, fails less often, and is excellent in maintainability. This invention also provides a method for preventing the occurrence of dents on a molten-metal-plated steel strip.
According to a first aspect of this invention, the apparatus includes a blade disposed to be pressed against a roll dipped in a molten metal plating bath, in which a metal strip travels. The blade scrapes away substances deposited on the surface of the roll. The apparatus also includes an arm for supporting the blade, and a screw member disposed above the plating bath, so as to traverse it and movable along the axis of the roll in the bath for removing substances deposited on the roll in the molten metal plating bath. Floats are mounted on the arm for adjusting the press force of the blade applied to the roll in the bath.
It is preferable that weights be mounted on the arm for adjusting the press force of the blade applied to the roll in the molten metal plating bath.
It is preferable that the width of the blade be from about one-tenth to about one-fourth of the barrel length of the roll.
It is preferable that the roll in the molten metal plating bath be a sink roll.
It is preferable that the molten metal is zinc containing aluminum and the metal strip is a steel strip.
According to a second aspect of this invention, there is provided a method for preventing the occurrence of dents on a molten-metal-plated metal strip when the metal strip travels and is continuously dipped into a molten metal plating bath and then pulled upward from the bath so that molten metal is plated on the surface of the metal strip. The method includes mounting the apparatus of the first aspect of the invention for removing substances deposited on the surface of a roll in the molten metal plating bath, and plating the metal strip by removing the substances deposited on the surface of the roll in the molten metal plating bath.
According to this invention, substances deposited on the surface of the roll (a support roll for preventing the vibration of the metal strip in addition to the sink roll) in the molten metal plating bath can be stably removed without problems, while a plating operation is carried out. As a result, dents conventionally produced on the metal strip having been plated can be reduced. Further, the effect that equipment cost is less expensive can be obtained because the apparatus is not complex in structure as compared with the structure of a conventional apparatus.